The present invention relates to heating devices for generating high temperatures.
It is known that high temperatures can be generated by passing an electrical current through a resistor. The maximum temperature thus attainable will be dependent on the material from which the heating element, i.e. the resistor, is made.
For very high temperatures, the heating elements are usually made of graphite.
Because graphite has a low electrical resistivity and in order to ensure that the resistance of the heating element is sufficient for the heat to be generated, the resistance of the heating element can be increased by either increasing its length or decreasing its cross-sectional area or both.
However, the mechanical strength and lifetime of the heating element can be decreased significantly if the chosen cross-sectional area is too small.
Therefore, the practical dimensions of the graphite heating element are selected in such a way that the element has sufficient electrical resistance and, at the same time, has an adequate mechanical strength.
In order to increase its length, and consequently its resistance, the graphite heating element can be in the form of a spiral.
High temperatures can also be generated by passing electrical current through a column consisting of lumps of coke. The resistance of the coke column is inversely proportional to the number and the area of contact points between the particles of coke as well as on the type of coke that is used.
The resistance of the coke column is significantly higher in comparison with, say, a bar of graphite of similar dimensions because the area of contact between the coke particles is small. The coke column has to be mechanically supported. This can be achieved by placing the coke column into a cylindrical vessel comprising of a refractory material such as alumina, magnesia etc. However, the maximum permissible temperature for the above-mentioned refractories is less than 2000 Celsius.
Theoretically, temperatures of the order of 4500 Celsius can be attained if graphite is used as the supporting vessel, instead of the vessel made of the above mentioned refractories. However, because graphite is a good conductor of electricity the electrical current will flow through the graphite vessel instead of through the coke, thus preventing attainment of the required temperature.
It is an object of the invention to provide a heating device which overcomes this difficulty.